The present invention pertains broadly to a system for processing a material through a zone which is marked by a controlled temperature along its length. The system is particularly applicable to crystallization, especially the formation of single crystals by freezing liquid melts or by solid state recrystallization of polycrystalline materials.
Crystals have applications in various fields, including semiconductors, thin films, and lasers. Several different materials fall under the term "crystal". Certain crystals may be amorphous, such as glass. Other crystals are called "polycrystals" because they actually include more than one crystal. Still others fall under the "single crystal" classification. Quality control of the latter is of particular concern, because of the need to minimize defects that would impair single crystal performance in industrial applications.
In a general aspect, single crystals can be formed by either freezing a liquid or by heating and cooling a polycrystalline mass. Defects in the resulting crystal or crystals often occur when the cooling is uncontrolled. For example, stresses may develop when forming the crystalline structure due to excessive temperature differences maintained along the length of the crystal during the cooling process.
Basically, crystal defects are departures in a crystalline solid from a regular array of atoms or ions. Natural crystals, for example, usually contain numerous defects due to the uncontrolled conditions under which they were formed. Defects which affect the color sometimes make these natural crystals valuable as gems. However, with synthetic crystals, defects in general are to be avoided.
The number of defects in a crystal can usually be reduced by cooling the crystal uniformly and slowly. One fairly common method of growing single crystals is called the Bridgeman-Stockbarger growth process, wherein the material is melted in a vertical cylindrical vessel which tapers conically to a point at the bottom. The vessel (ampoule) is lowered slowly into a cold zone, i.e., a zone having a temperature below the melting point. Crystallization begins in the tip and continues usually by growth from the first formed nucleus.
A modified version of this method is discussed in D. Cox & F. Fong, "Growth of Single Crystals of Anhydrous Lanthanide Halides," J. Crystal Growth, Vol. 20, 1973 pp. 233-238, hereby incorporated by reference. This procedure employs a furnace having dual temperature zones. A hot zone is established at one end of the furnace, and a lower temperature zone is established at the other end. The feed material is crystallized as it is lowered at a controlled rate from the hot zone to the cooler zone. The temperature gradient near the crystal melting temperature is particularly important in the crystal growth process.
The dual temperature furnace, discussed by Cox and Fong, may be constructed by attaching a high temperature heating element, e.g., a coil, at one end of a ceramic oven or furnace and a lower temperature heating element at the opposite end. Because heat is allowed to escape at both ends, a rough thermal gradient along the length of the furnace can be established. Nevertheless, the temperature of the furnace generally needs to be closely monitored to adequately control the gradient. This can involve complex temperature sensing circuitry and adjustments of the electrical power input. More conventionally, furnaces do not provide for precise temperature control of the gradient Moreover, most furnaces possess large thermal masses, resulting in long equilibration times.
Accordingly, there exists a continuing need for a material processing system which is capable of readily reaching and maintaining a desired equilibrium temperature gradient or profile. It is further desirable that the system be relatively simple and dependable. The need for such a system exists especially in crystallization operations; however, similar needs are evident in chemical reactions and other important industrial operations.